Thrust bearing and bearing device for crankshaft of internal combustion engine

ABSTRACT

A thrust bearing for receiving an axial force of a crankshaft of an internal combustion engine. The thrust bearing includes a pair of half thrust bearings having a semi-annular shape that form an annular shape by abutting their circumferential end surfaces. Each half thrust bearing include thrust reliefs formed adjacent to both circumferential end portions of a sliding surface receiving the axial force so that a wall thickness is thinner toward the circumferential end surface. A thrust relief length L 2  at an inner end portion of the thrust relief positioned on a rear side of one of the half thrust beings in a rotational direction is larger than a thrust relief length L 1  at an inner end portion of the thrust relief positioned on a front side of the other of the half thrust bearing in the rotational direction.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to a thrust bearing and a bearing devicethat receive an axial force of a crankshaft of an internal combustionengine.

(2) Description of Related Art

A crankshaft of an internal combustion engine is rotatably supported, atits journal portion, by a lower part of a cylinder block of an internalcombustion engine via a main bearing which is configured by assembling apair of half bearings into a cylindrical shape.

One or both of the pair of half bearings is used to be combined with ahalf thrust bearing which receives an axial force of the crankshaft. Thehalf thrust bearing is provided at one or both of axial end surfaces ofthe half bearing.

The half thrust bearing receives the axial force generated in thecrankshaft. That is, the half thrust bearing is placed for the purposeof bearing the axial force applied to the crankshaft when the crankshaftis connected to a transmission via a clutch, for example.

On a sliding surface side of the half thrust bearing close to bothcircumferential ends thereof, thrust reliefs are formed so that abearing member becomes thinner toward the respective circumferential endsurfaces. In general, the thrust relief is formed so that a length fromthe circumferential end surface to the sliding surface of the halfthrust bearing, and a depth at the circumferential end surface areconstant irrespective of the position in a radial direction. The thrustrelief is formed in order to absorb misalignment of the end surfaces ofthe pair of half thrust bearings when mounting the half thrust bearingson a split type bearing housing (see FIG. 10 of JP-A-11-201145).

The crankshaft of the internal combustion engine is supported, at itsjournal portion, by the lower part of the cylinder block of the internalcombustion engine via the main bearing consisting of the pair of halfbearings. Here, lubrication oil is fed from an oil gallery in a wall ofthe cylinder block through a through hole in a wall of the main bearinginto a lubrication oil groove formed along an inner circumferentialsurface of the main bearing. The lubrication oil is supplied to thelubrication oil groove of the main bearing in this manner, and thensupplied to the half thrust bearings.

Meanwhile, in recent years, oil pumps for supplying lubrication oil ofinternal combustion engines have been reduced in size, so that theamount of lubrication oil to be supplied to the bearings decreases.Accordingly, the amount of lubrication oil leaking through the endsurfaces of the main bearing tends to decrease, and the amount oflubrication oil supplied to the half thrust bearings also tends todecrease. To address this, there is proposed an art for improvingretainability of lubrication oil by forming a plurality of narrowgrooves in parallel on the sliding surfaces of the half thrust bearings,for example (see JP-A-2001-323928).

BRIEF SUMMARY OF THE INVENTION

Further, in recent years, crankshafts have been reduced in diameter forreducing the weight of internal combustion engines, so that the rigidityof the crankshafts becomes low. Accordingly, the crankshaft tends todeform easily during operation of the internal combustion engine, andthe vibration of the crankshaft tends to become large. As a result, thesliding surface near a circumferentially central portion of the halfthrust bearing tends to directly contact with a thrust collar surface ofthe crankshaft, so that damage (seizure) easily occurs.

JP-A-2001-323928 discloses a configuration in which the oil grooves areprovided on the sliding surface to supply the lubrication oil to almostthe entire bearing surface. However, even if adopting the art ofJP-A-2001-323928, it is difficult to prevent the sliding surface,particularly adjacent to the thrust relief on a rear side of the halfthrust bearing in a crankshaft rotational direction from contacting withthe thrust collar of the crankshaft if the vibration of the crankshaftdue to the deformation is large as above. Therefore, there has been apossibility that seizure occurs in the sliding surface of the halfthrust bearing.

Accordingly, an object of the present invention is to provide a thrustbearing and a bearing device in which the seizure hardly occurs duringoperation of the internal combustion engine.

According to the present invention, there is provided a thrust bearing(10) for receiving an axial force of a crankshaft of an internalcombustion engine which rotates in one direction, the thrust bearingincluding a pair of half thrust bearings (8, 8) having a semi-annularshape, the pair of half thrust bearings forming an annular shape byabutting respective circumferential end surfaces (83, 83) thereof,wherein

the half thrust bearing includes thrust reliefs (82F, 82R) formedadjacent to both circumferential end portions of a sliding surface (81)receiving the axial force so that a wall thickness of the half thrustbearing is thinner toward the circumferential end surface, and

a thrust relief length (L2) at an inner end portion of the thrust relief(82R) positioned on a rear side of one of the half thrust bearing in acrankshaft rotational direction is larger than a thrust relief length(L1) at an inner end portion of the thrust relief (82F) positioned on afront side of the other of the half thrust bearings in the crankshaftrotational direction.

Here, the crankshaft is a member including a journal portion, a crankpinportion, and a crank arm portion. While the half thrust bearing is amember having a shape obtained by dividing an annular shape intoapproximately halves, it is not intended to be strictly half.

The thrust bearing of the present invention is a thrust bearing which isformed in an annular shape by combining a pair of half thrust bearingshaving a semi-annular shape that receive an axial force of a crankshaftof an internal combustion engine. A thrust relief length of a thrustrelief positioned on a rear side in a crankshaft rotational direction isformed so as to be longer than that of a thrust relief positioned on afront side of the half thrust bearing in the rotational direction. Bythis configuration, lubrication oil flowing out of crush reliefs of apair of half bearings supporting a journal portion of the crankshafteasily flows into a thrust relief clearance formed by the thrust reliefpositioned on the rear side in the crankshaft rotational direction andthe thrust relief positioned on the front side in the crankshaftrotational direction, and thus a large amount of the lubrication oil isfed to the sliding surface of the thrust bearing. Accordingly, even whenthe deformation of the crankshaft is generated during the operation ofthe internal combustion engine and therefore the vibration becomeslarge, a sliding surface of the thrust bearing hardly comes into directcontact with a thrust collar surface of the crankshaft, so that damageof the sliding surface of the half thrust bearing hardly occurs.Further, According to this configuration, it is possible to maintaininga high load capability since the area of the sliding surface of thethrust bearing is not reduced.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is an exploded perspective view of a bearing device;

FIG. 2 is a front view of a thrust bearing of embodiment 1;

FIG. 3 is a side view of thrust reliefs of the thrust bearing in FIG. 2seen in the direction of arrow Y1;

FIG. 4 is a front view of half bearings and the thrust bearing;

FIG. 5 is a cross-sectional view of the bearing device;

FIG. 6 is a front view of a thrust bearing of embodiment 2;

FIG. 7 is a front view of a thrust bearing of embodiment 3;

FIG. 8 is an end view of an upper half thrust bearing in FIG. 7 seen inthe direction of arrow Y2, illustrating a circumferential end surface ofa thrust relief positioned on a rear side in a crankshaft rotationaldirection;

FIG. 9 is an end view of a lower half thrust bearing in FIG. 7 seen inthe direction of arrow Y3, illustrating a circumferential end surface ofa thrust relief positioned on a front side in the crankshaft rotationaldirection;

FIG. 10 is a front view of a thrust bearing of embodiment 4;

FIG. 11 is a side view of a thrust relief positioned on a rear side ofan upper half thrust bearing in FIG. 10 in the crankshaft rotationaldirection, seen in the direction of arrow Y4;

FIG. 12 is a side view of a thrust relief positioned on a front side ofa lower half thrust bearing in FIG. 10 in the crankshaft rotationaldirection, seen in the direction of arrow Y4;

FIG. 13A is a side view of a thrust relief positioned on a rear side ofa half thrust bearing in the crankshaft rotational direction of anotherembodiment;

FIG. 13B is a side view of a thrust relief positioned on a rear side ofa half thrust bearing in the crankshaft rotational direction of anadditional embodiment;

FIG. 14 is a front view of a half thrust bearing of another embodiment;

FIG. 15 is a front view of thrust reliefs of another embodiment;

FIG. 16 is a front view of a thrust bearing of another embodiment;

FIG. 17A is a front view of half bearings and a thrust bearing forexplaining operation of the embodiment;

FIG. 17B is a view showing an inner side of the half bearings and thethrust bearing in FIG. 17A as seen from the inside in a radialdirection;

FIG. 18 is a front view of a half bearing of embodiment 5; and

FIG. 19 is a bottom view of the half bearing in FIG. 18 as seen from theinside in a radial direction.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described withreference to the drawings.

Embodiment 1

(General Configuration of Bearing Device)

First, a description will be given for a general configuration of abearing device 1 having a thrust bearing 10 of the present invention,using FIGS. 1, 4, and 5. As shown in FIGS. 1, 4, and 5, a bearinghousing 4 configured by attaching a bearing cap 3 to a lower part of acylinder block 2 has a bearing hole 5 which is a circular holepenetrating between side surfaces of the cylinder block 2 and thebearing cap 3. The side surfaces have seats 6 and 6, which are annularrecesses, on the periphery of the bearing hole 5. Half bearings 7 and 7are assembled in a cylindrical shape and fitted into the bearing hole 5.The half bearings 7 and 7 rotatably support a journal portion 11 of acrankshaft which rotates in one direction. Half thrust bearings 8 and 8are assembled in an annular shape and fitted into the seats 6 and 6. Thehalf thrust bearings 8 and 8 receive an axial force f (see FIG. 5) via athrust collar 12 of the crankshaft.

As shown in FIG. 4, a lubrication oil groove 71 is formed on an innercircumferential surface of the half bearing 7 (the upper one) on a sideof the cylinder block 2 of the half bearings 7 constituting a mainbearing, and a through hole 72 is formed to penetrate the half bearing 7from the lubrication oil groove 71 to an outer circumferential surface(also see FIGS. 18 and 19). It is to be noted that the lubrication oilgroove may be formed on both of the upper and lower half bearings.

Further, each half bearing 7 has crush reliefs 73 and 73 on both endsthereof adjacent to abutment surfaces of the half bearings 7 (see FIG.4). The crush reliefs 73 are wall-thickness reduction regions formed sothat a wall thickness of a region adjacent to circumferential endsurfaces of the half bearing 7 is gradually reduced toward thecircumferential end surface. The crush reliefs 73 are formed forabsorbing misalignment and deformation of abutment surfaces whenassembling the pair of half bearings 7 and 7.

(Configuration of Thrust Bearing)

Next, the configuration of the thrust bearing 10 of embodiment 1 will bedescribed with reference to FIGS. 2 to 5. The thrust bearing 10 ofembodiment 1 is formed by combining a pair of half thrust bearings 8 inan annular shape. Each half thrust bearing 8 is a semi-annular shapedflat plate formed from a bimetal obtained by bonding a thin bearingalloy layer to a back metal layer. Each half thrust bearing 8 includes asliding surface 81 (bearing surface) at a central region that is asurface configured by the bearing alloy layer, and thrust reliefs 82Rand 82F at regions adjacent to end surfaces 83 and 83 on bothcircumferential sides of the half thrust bearing 8. In order to improveretainability of the lubrication oil, two oil grooves 81 a and 81 a areformed between the opposite thrust reliefs 82R and 82F on the slidingsurface 81.

The thrust reliefs 82R and 82F are wall-thickness reduction regionsformed adjacent to end surfaces of the sliding surface 81 on bothcircumferential sides so that the wall thickness is gradually reducedtoward the end surface of the half thrust bearing 8. The thrust reliefs82R and 82F are formed over the entire radial length of thecircumferential end surfaces 83 of the half thrust bearing 8. The thrustreliefs 82R and 82F are formed to alleviate misalignment between thecircumferential end surfaces 83 and 83 of the pair of thrust bearings 8and 8, that may be caused by misalignment and the like between endsurfaces of the respective split type bearing housings 2 and 3 whenmounting the half thrust bearings 8 on the split type bearing housing 4.

The surfaces of the thrust reliefs 82R and 82F are configured by flatsurfaces in embodiment 1, but may be configured by curved surfaces. Inthe case of flat surfaces, the surfaces may be configured by a singleflat surface, or by combining a plurality of flat surfaces. Also in thecase of curved surfaces, the surface may be configured by a singlecurved surface, or by combining a plurality of curved surfaces. Further,the surface of each thrust reliefs 82R and 82F may be configured bycombining a flat surface and a curved surface (FIG. 11).

As will be understood from FIGS. 2 and 4, when the half thrust bearing 8of the embodiment is mounted on the bearing housing 4 of the bearingdevice 1, a thrust relief length L2 at an inner end portion of thethrust relief 82R on a side of the circumferential end surface 83 whichis assembled so as to be positioned on a rear side in the rotationaldirection of the crankshaft rotating in one direction (hereinafter,referred to as “rear side thrust relief”) is formed to differ from, andlonger than a thrust relief length L1 at an inner end portion of thethrust relief 82F on a side of the circumferential end surface 83positioned on a front side in the crankshaft rotational direction(hereinafter, referred to as “front side thrust relief”). Here, inembodiment 1, the thrust relief length of the rear side thrust relief82R and the thrust relief length of the front side thrust relief 82F areformed to be constant between the inner end portion and an outer endportion.

Here, the “thrust relief length” means a length measured vertically froma plane (imaginary dividing plane H) passing through both end surfaces83 and 83 of the one half thrust bearing 8 to the other end of thethrust relief. More specifically, the thrust relief lengths L1 and L2 atthe inner end portion are respectively defined as lengths from theimaginary dividing plane H to the points where the surfaces of thethrust reliefs 82F and 82R intersect the inner circumferential edge ofthe sliding surface 81.

Further, the “thrust relief positioned on a rear side in the crankshaftrotational direction” means the thrust relief 82R of the thrust reliefsat both circumferential end portions of one half thrust bearing 8, thatfirstly passes through any point on the thrust collar 12 of the rotatingcrankshaft, and the “thrust relief positioned on a front side in thecrankshaft rotational direction” means the thrust relief 82F whichsecondly passes through the point.

Meanwhile, the crankshaft of an internal combustion engine rotates inone direction during operation. Accordingly, those skilled in the artwould recognize which one of the two thrust reliefs adjacent to bothcircumferential end surfaces of the half thrust bearing is the “thrustrelief positioned on a rear side in the crankshaft rotationaldirection”, taking the crankshaft rotational direction intoconsideration. In addition, those skilled in the art would be able todesign and manufacture the thrust bearing 10 of the present invention inaccordance with the disclosure of the present invention, and to mount iton a bearing device supporting a crankshaft rotating in one direction.

FIG. 3 shows a side near the abutment portion of the circumferential endsurfaces 83 and 83 of the pair of half thrust bearings 8 seen from theinner side (in the direction of arrow Y1 in FIG. 2). It is to be notedthat the abutment portion near the other circumferential end surfaces 83and 83 of the pair of half thrust bearings 8 on the right side in FIG. 2also has a configuration similar to that in FIG. 3.

Each half thrust bearing 8 has a constant thickness at the portion ofthe sliding surface 81, and the pair of half thrust bearings 8 and 8have the same thickness t at the portion of their sliding surfaces 81 asshown in FIG. 3. The surfaces of the front side thrust relief 82F andthe rear side thrust relief 82R of the half thrust bearing 8 have thesame thrust relief depth RD (FIG. 3) at the circumferential endsurfaces. In embodiment 1, the thrust relief depth RD of the front sidethrust relief 82F and the rear side thrust relief 82R at thecircumferential end surfaces is constant between the inner end portionand the outer end portion.

Here, the “thrust relief depth” means an axial distance from a planecontaining the sliding surface 81 of the half thrust bearing 8 to thesurface of the thrust relief. In other words, the thrust relief depth isa distance measured vertically from an imaginary sliding surfaceobtained by extending the sliding surface 81 over the thrust reliefs 82Fand 82R. Hence, the thrust relief depth RD at the circumferential endsurface is defined as an axial distance from the surface of the thrustrelief at the circumferential end surface of the half thrust bearing 8to the imaginary sliding surface.

As specific dimensions of the front side thrust relief 82F and the rearside thrust relief 82R, the thrust relief depth RD at thecircumferential end surfaces 83 and 83 of the half thrust bearing 8 fromthe sliding surface 81 to the front side thrust relief 82F and the rearside thrust relief 82R may be 0.3 mm to 1.0 mm, for example, when thethrust bearing is used for a crankshaft of a small internal combustionengine for a passenger car and the like (in which a journal portiondiameter is about 30 to 100 mm). It is to be noted that the front sidethrust relief 82F and the rear side thrust relief 82R have the samethrust relief depth RD. The thrust relief length L1 of the front sidethrust relief 82F at the inner end portion may be 1 mm to 5 mm, and thethrust relief length L2 of the rear side thrust relief 82R at the innerend portion may be 5 mm to 25 mm. It is to be noted that the abovedimensions are merely examples, and the dimensions are not limited tothese ranges.

The foregoing numerical ranges of the thrust relief length L1 of thefront side thrust relief 82F and the thrust relief length L2 of the rearside thrust relief 82R are partially overlapped with each other.However, this is because the larger the bearing is, the longer thethrust relief length needs to be. Accordingly, the thrust relief lengthsL1 and L2 should satisfy the relation L2>L1 within their respectivenumerical ranges.

In addition, as shown in FIG. 7, the rear side thrust relief ispreferably formed within a range of a circumferential angle θ1 of 25° orless from the circumferential end surface 83 of the half thrust bearing8, and a pair of the rear side thrust relief and the front side thrustrelief is preferably formed within a range of a circumferential angle θ2of 30° or less.

(Operation)

Next, the operation of the thrust bearing 10 of the embodiment will bedescribed, using FIGS. 4, 5, 17A, and 17B.

(Oil Supply Operation)

In the bearing device 1, the lubrication oil discharged by pressurizedfrom an oil pump (not shown) passes an internal oil passage of thecylinder block 2 and the through hole 72 penetrating the wall of thehalf bearing 7, and is supplied to the lubrication oil groove 71 on theinner circumferential surface of the half bearing 7. The lubrication oilsupplied into the lubrication oil groove 71 is partly supplied to theinner circumferential surface of the half bearing 7, partly fed throughan opening (not shown) for an internal oil passage of the crankshaftprovided on a surface of the journal portion toward a crankpin, andfurther, partly passes through a clearance between the surface of thecrush reliefs 73 of the pair of half bearings 7 and 7 forming the mainbearing and the surface of the journal portion 11 of the crankshaft toflow from both axial ends of the half bearings 7 and 7 to the outside.

In the embodiment, the half bearings 7 are arranged concentrically withthe half thrust bearings 8, and a plane containing both circumferentialend surfaces of the half bearings 7 constituting the main bearing ismade parallel to a plane containing both circumferential end surfaces ofthe half thrust bearings 8, so that the positions of the crush reliefs73 correspond to those of the thrust reliefs 82F and 82R.

Hereinafter, the operation of the present invention will be described.

Immediately after flowing out of the clearance between the crush reliefsof the half bearings 7, the lubrication oil flows toward a front side inthe crankshaft rotational direction from the position of the abutmentportion (joint portion) of a circumferential end surface of one halfthrust bearing 8 and that of the other half thrust bearing 8 as shown inFIGS. 17A and 17B (dashed arrow) due to an inertial force causing theoil to move on the front side in the rotational direction of thecrankshaft 11 because the lubrication oil has circumferentially flownwith the surface of the journal portion of the rotating crankshaft.

The thrust bearing 10 of the embodiment includes the pair of half thrustbearings 8 and 8, and the thrust relief length L2 of the rear sidethrust relief 82R of each half thrust bearing 8 is longer than thethrust relief length L1 of the front side thrust relief 82F. Accordingto the configuration, the center of the length at the inner end portionof a thrust relief clearance S surrounded by the surface of the frontside thrust relief 82F of one half thrust bearing 8, the surface of therear side thrust relief 82R of the other half thrust bearing 8, and theimaginary sliding surface obtained by extending the sliding surface 81over the surfaces of the front side thrust relief 82F and the rear sidethrust relief 82R (see FIG. 17B) is positioned on a front side in therotational direction of the crankshaft 11 relative to the abutmentportion (joint portion) of the circumferential end surface 83 of onehalf thrust bearing 8 and that of the other half thrust bearing 8.Accordingly, the lubrication oil flown out of the crush relief clearanceof the half bearings 7 and then flowing on the front side in therotational direction of the crankshaft 11 relative to the position ofthe abutment portion (joint portion) of the circumferential end surfaces83 and 83 of the pair of half thrust bearings 8 flows into the thrustrelief clearance S in large amounts, and is then fed to the slidingsurface 81.

During the operation of the internal combustion engine, the deformationof the crankshaft (deflection in the axial direction) occurs especiallyunder the operation condition in which the crankshaft is rotated at highspeeds, and thereby the vibration of the crankshaft becomes large. Thelarge vibration periodically causes an axial force f in the crankshafttoward the sliding surface 81 of the thrust bearing 10. The slidingsurface 81 of the thrust bearing 10 receives the axial force f.

In the thrust bearing 10 of the present invention, even under the axialforce f of the crankshaft, the lubrication oil flown out of the crushrelief clearance between the pair of half bearings 7 easily flows intothe thrust relief clearance S in large amounts that is formed by thethrust relief 82F positioned on a front side in the rotational directionand the thrust relief 82R positioned on a rear side in the rotationaldirection in the thrust bearing 10, and this large amount of lubricationoil is fed to the sliding surface 81 as described above. Since the largeamount of lubrication oil is supplied, the sliding surface 81 of thethrust bearing 10 hardly comes into direct contact with the surface ofthe thrust collar 12 of the crankshaft.

In the pair of half thrust bearings 8 constituting the thrust bearing 10of the embodiment, the thrust relief length L2 of the rear side thrustrelief 82R is longer than the thrust relief length L1 of the front sidethrust relief 82F. When comparing the configuration of the embodimentwith the configuration of a conventional thrust bearing in which thrustreliefs with the same length are formed at both circumferential endportions, the center of the length at a radially inner end portion ofthe thrust relief clearance formed by the thrust reliefs with the samelength between the pair of half thrust bearings constituting theconventional thrust bearing is positioned at an abutment portion (jointportion) of circumferential end surfaces of the pair of half thrustbearings, while the center of the length at the radially inner endportion of the thrust relief clearance S in the configuration of theembodiment is shifted on a front side in the crankshaft rotationaldirection relative to the abutment portion (joint portion) of thecircumferential end surfaces without reducing the area of the slidingsurface 81. Accordingly, the thrust bearing 10 of the present inventionstill maintains a high ability of receiving (bearing) the axial load ofthe crankshaft. In contrast, in the case of the conventional thrustbearing, if the lengths of the thrust reliefs on both circumferentialsides of the half thrust bearing are increased for increasing the amountof lubrication oil flowing out of the crush relief clearance of the mainbearing (half bearings), the area of the sliding surface of the thrustbearing has to be reduced, and accordingly the ability of the thrustbearing of receiving the axial load of the crankshaft is decreased.

Additionally, in the pair of half thrust bearings 8 and 8 constitutingthe thrust bearing 10 of the embodiment, the thrust relief depths RD ofthe front side thrust relief 82F and the rear side thrust relief 82R aremaximum and have the corresponding thickness at the circumferential endsurfaces so as to correspond to each other (so as not to cause a step)over the entire length in the radial direction. Accordingly, theoriginal effect of the thrust relief of alleviating the misalignmentbetween the circumferential end surfaces 83 and 83 of the pair of halfthrust bearings 8 and 8, which effect is caused by the misalignment orthe like of the end surfaces of the split type bearing housings 2 and 3when assembling the pair of half thrust bearings 8 and 8 into the splittype bearing housing 4, is not affected.

Embodiment 2

Hereinafter, a description will be given, using FIG. 6, for a thrustbearing 10A constituted by a pair of half thrust bearings 8A each havinga front side thrust relief 82F and a rear side thrust relief 82R whichare another than those in embodiment 1. The parts identical or equal tothose in embodiment 1 will be described with the same referencecharacters.

(Configuration)

The configuration will be described first. The configuration of thethrust bearing 10A according to the embodiment is generally similar tothat in embodiment 1 except for the shapes of the front side thrustrelief 82F and the rear side thrust relief 82R of the pair of halfthrust bearings 8A constituting the thrust bearing 10A.

Specifically, a surface of the rear side thrust relief 82R of the halfthrust bearing 8A of the embodiment is constituted by a flat surface,and as shown in FIG. 6, a thrust relief length L2 is configured so as tobe minimum (L2I) at the radially inner end portion and maximum (L2O) atthe radially outer end portion. A surface of the front side thrustrelief 82F is also constituted by a flat surface, and as shown in FIG.6, a thrust relief length L1 is configured so as to be maximum (L1I) atthe radially inner end portion and minimum (L1O) at the radially outerend portion.

On the other hand, other relations such as the relation between thethrust relief lengths L1I and L2I at the inner end portion and therelation between the thrust relief depths RD of the front side thrustrelief and the rear side thrust relief are similar to those inembodiment 1.

The other configurations of embodiment 2 are generally similar to thosein embodiment 1 and therefore, embodiment 2 has operations and effectssimilar to those in embodiment 1.

Embodiment 3

Hereinafter, a description will be given, using FIGS. 7 to 9, for athrust bearing 10B constituted by a pair of half thrust bearings 8B eachhaving a front side thrust relief 82F and a rear side thrust relief 82Rwhich are another than those in embodiments 1 and 2. The parts identicalor equal to those in embodiments 1 and 2 will be described with the samereference characters.

(Configuration)

The configuration will be described first. The configuration of thethrust bearing 10B according to embodiment 3 is generally similar tothat in embodiment 1 except for the shapes of the front side thrustrelief 82F and the rear side thrust relief 82R of the pair of halfthrust bearings 8B constituting the thrust bearing 10B.

Specifically, a surface of the rear side thrust relief 82R of each halfthrust bearing 8B of the embodiment is constituted by a flat surface,and as shown in FIG. 7, a thrust relief length L2 is configured so as tobe minimum at the radially inner end portion and maximum at the radiallyouter end portion. A surface of the front side thrust relief 82F is alsoconstituted by a flat surface, and as shown in FIG. 7, a surface ofthrust relief length is configured so as to be minimum at the radiallyinner end portion and maximum at the radially outer end portion.

FIG. 8 shows the circumferential end surface 83 of the upper half thrustbearing 8B positioned on a rear side in the crankshaft rotationaldirection, seen in the direction of arrow Y2 in FIG. 7, and FIG. 9 showsthe circumferential end surface 83 of the lower half thrust bearing 8Bpositioned on a front side in the crankshaft rotational direction, seenin the direction of arrow Y3 in FIG. 7. As shown in FIGS. 8 and 9, inthe half thrust bearing 8B of the embodiment, unlike embodiments 1 and2, the rear side thrust relief 82R and the front side thrust relief 82Fhave the thrust relief depths RD measured from the sliding surface 81that are maximum (RD1) at the inner end portions and minimum (RD2) atthe outer end portions, at the circumferential end surfaces 83 and 83.Also in this case, the thrust relief depths RD of the front side thrustrelief 82F and the rear side thrust relief 82R correspond to each otherover the entire radial length of the circumferential end surfaces 83. Itis to be noted that the front side thrust relief 82F of the upper thrustbearing 8B and the rear side thrust relief 82R of the lower half thrustbearing 8B shown in FIG. 7 are also configured in the same manner.

As specific dimensions of the thrust reliefs 82R and 82F, the thrustrelief depths RD1 at the inner end portions and the thrust relieflengths L1 and L2 of the thrust reliefs 82R and 82F are similar to thosein embodiment 1, while the thrust relief depths RD2 at the outer endportions may be 0.15 to 0.5 mm when used for a crankshaft (in which ajournal portion diameter is about 30 to 100 mm) of a small internalcombustion engine for a passenger car and the like. The above dimensionsare, however, merely examples, and the dimensions are not limited tothese ranges.

In embodiment 3, the lubrication oil having flown into the thrustreliefs 82R and 82F hardly flows to the outside through the radiallyouter end portions of the thrust reliefs 82R and 82F, and becomes easilysupplied to the sliding surface. The other configurations are generallysimilar to those in embodiment 1 and therefore embodiment 3 hasoperations and effects similar to those in embodiment 1.

Embodiment 4

Hereinafter, a description will be given, using FIGS. 10 to 12, for athrust bearing 10C constituted by a pair of half thrust bearings 8C eachhaving a front side thrust relief 82F and a rear side thrust relief 82Rwhich are another than those in embodiments 1-3. The parts identical orequal to those in embodiments 1-3 will be described with the samereference characters.

(Configuration)

The configuration will be described first. The configuration of thethrust bearing 10C according to the embodiment is generally similar tothat in embodiment 1 except for the shapes of the front side thrustrelief 82F and the rear side thrust relief 82R of the pair of halfthrust bearings 8C constituting the thrust bearing 10C.

FIG. 11 shows a side view of an area near the circumferential endsurface 83 of the half thrust bearing 8C positioned on a rear side inthe crankshaft rotational direction, seen from the inside (in thedirection of arrow Y4 in FIG. 10), and FIG. 12 shows a side view of anarea near the circumferential end surface 83 of the half thrust bearing8C positioned on a front side in the crankshaft rotational direction,seen from the inside (in the direction of arrow Y4 in FIG. 10).

As shown in FIG. 10, in the half thrust bearing 8C of embodiment 4, asurface of the rear side thrust relief 82R is configured by a flatsurface adjacent to the sliding surface 81 and a curved surface(arc-shaped surface) which is adjacent to the circumferential endsurface 83 of the half thrust bearing 8C and is depressed from thesliding surface 81 toward an opposite surface (back face) to be aconcave shape, and a thrust relief length L2 is configured so as to beminimum (L2I) at the inner end portion and becomes longer toward theouter end portion.

A surface of the front side thrust relief 82F is configured by a curvedsurface (arc-shaped surface) depressed from the sliding surface 81toward the opposite surface (back face) to be a concave shape, and athrust relief length L1 is configured so as to be constant between theinner end portion and the outer end portion as shown in FIG. 10.

The relation between the thrust relief length L2 of the rear side thrustrelief 82R and the thrust relief length L1 of the front side thrustrelief 82F at the inner end portions are similar to that in embodiment1.

A thrust relief depth RD of the front side thrust relief 82F at thecircumferential end surface 83 and a thrust relief depth RD of the rearside thrust relief 82R at the circumferential end surface 83 areconstant over the radial entire length of the thrust reliefs, and aremade to correspond to each other. The front side thrust relief 82F andthe rear side thrust relief 82R shown on the right side in FIG. 10 arealso configured in the same manner.

A length L2′ of the curved surface portion of the rear side thrustrelief 82R shown in FIG. 11 (a length measured vertically from a planecontaining the circumferential end surface 83 of the half thrust bearing8C) is generally the same as a thrust relief length L1 of the front sidethrust relief 82F shown in FIG. 12, and constant between the innerdiameter end portion and the outer diameter end portion. The length L2′of the curved surface portion of the rear side thrust relief 82R at theinner end portion may be 0.5 to 3.5 mm.

The curved surface portion of the rear side thrust relief 82R is formedso that a depth RD′ from the sliding surface 81 to a position where thecurved surface portion is connected to the flat surface portion isconstant over the entire radial length of the thrust relief. The depthRD′ of the curved surface portion of the rear side thrust relief 82Rfrom the sliding surface 81 to a position where the curved surfaceportion is connected to the flat surface portion is preferably 0.005 mmto 0.1 mm.

As shown in FIGS. 11 and 12, the curved surface portion of the rear sidethrust relief 82R and the curved surface of the front side thrust relief82F may be an arc-shaped surface having the same radius of curvature R.However, the curved surface portion of the rear side thrust relief 82Rand the curved surface of the front side thrust relief 82F are notlimited to this, and may be an elliptical arc surface or a free curvedsurface (free-form surface).

The length L2′ of the curved surface portion of the rear side thrustrelief 82R is not limited to the embodiment, and may be configured sothat the length of the curved surface portion becomes longer or shorterfrom the inner end portion toward the outer end portion of the halfthrust bearing.

Also, the flat surface portion of the rear side thrust relief 82R shownin FIG. 11 close to the sliding surface 81 may be changed to a curvedsurface having a radius of curvature R2 and bulging from the back faceside toward the sliding surface 81 side to be a convex shape, as shownin FIG. 13A. Although FIG. 13A shows a curved surface having the radiusof curvature R2, it would be understood that the curved surface portionis not limited to this and may have an elliptical arc surface or a freecurved surface.

Similarly, the curved surface portion of the rear side thrust relief 82Rshown in FIG. 11 or 13 adjacent to the circumferential end surface 83may be changed to a flat surface (FIG. 13B). Further, the curved surfaceportion or the flat surface portion of the rear side thrust relief 82Radjacent to the circumferential end surface 83 may be configured so thata depth RD′ measured from the sliding surface 81 at the position wherethe curved surface portion or the flat surface portion is connected tothe curved surface portion or the flat surface portion close to thesliding surface 81 becomes deeper or shallower from the inner endportion toward the outer end portion of the half thrust bearing 8C.

The surface of the front side thrust relief 82F is preferably configuredsimilarly to the curved surface portion or the flat surface portion ofthe rear side thrust relief 82R adjacent to the circumferential endsurface 83.

Embodiment 5

Next, the bearing device 1 equipped with the thrust bearing of thepresent invention will be described using FIGS. 2 to 5, 18, and 19. Theparts identical or equal to those in the foregoing embodiments will bedescribed with the same reference characters.

In the embodiment, a description will be given for the bearing device 1equipped with the thrust bearing 10 described in embodiment 1, althoughthe bearing device is not limited to this. It should be noted that thebearing device 1 with any of the thrust bearings 10A to 10C ofembodiments 2-4 also has an operation and an effect similar to thatbelow.

As shown in FIGS. 4 and 5, the bearing device 1 of the embodimentincludes the bearing housing 4 having the cylinder block 2 and thebearing cap 3, two half bearings 7 and 7 for rotatably supporting thejournal portion 11 of the crankshaft, and two thrust bearings 10 (fourhalf thrust bearings 8) for receiving the axial force via the thrustcollar 12 of the crankshaft.

The cylinder block 2 and the bearing cap 3 constituting the bearinghousing 4 have a bearing hole 5 penetrating therethrough at their jointportion as a retaining hole for retaining the pair of half bearings 7and 7.

Each half bearing 7 has crush reliefs 73 and 73 at both circumferentialend portions on its inner circumferential surface. As shown in FIGS. 18and 19, the half bearing 7 placed on a cylinder block 2 side includes alubrication oil groove 71 formed along the circumferential direction andnear the center portion in a width direction (axial direction), and athrough hole 72 penetrating the half bearing 7 to extend from thelubrication oil groove 71 on the inner circumferential surface to itsouter circumferential surface.

Axially opposite sides of the pair of half bearings 7 and 7 are eachprovided with the thrust bearing 10 constituted by the pair of halfthrust bearings 8 and 8. Each half thrust bearing 8 is formed in asemi-annular shape. The half bearing 7 and the half thrust bearing 8 areplaced such that their outer diameters are approximately concentric toeach other and a horizontal plane passing through both circumferentialend surfaces of the half bearing 7 and a horizontal plane passingthrough both circumferential end surfaces of the half thrust bearing 8are approximately parallel to each other.

As a result, the crush relief 73 of the half bearing 7 and the thrustreliefs 82 of the half thrust bearing 8 correspond one-to-one to eachother as shown in FIG. 4.

As described in embodiment 1, in the half thrust bearing 8, the thrustrelief length L2 of the rear side thrust relief 82R at the inner endportion is longer than the thrust relief length L1 of the front sidethrust relief 82F at the inner end portion.

Accordingly, in the embodiment, as shown in FIG. 4, the center positionof the length at the inner end portion of the thrust relief clearance Sformed by the front side thrust reliefs 82F and the rear side thrustreliefs 82R at the abutment portion of both circumferential ends of thepair of half thrust bearings 8 (namely, the clearance surrounded by thesurface of the front side thrust relief 82F, the surface of the rearside thrust relief 82R, and the imaginary extended surface of thesliding surface 81, seen from the inside of the thrust bearing 10 in theradial direction) is shifted on a front side in the crankshaftrotational direction relative to the center position of thecorresponding crush relief clearance surrounded by the surfaces of thecrush reliefs 73 of the pair of half bearings 7 and the imaginaryextended surface of an inner circumferential surface 75 of the halfbearing 7.

In addition to the relation between the thrust relief length L1 and thethrust relief length L2, the pair of half thrust bearings 8 constitutingthe thrust bearing 10 of the embodiment has the following relation tothe half bearing 7.

That is to say, in the half thrust bearing 8 of the embodiment, thethrust relief length L2 of the rear side thrust relief 82R at the innerend portion is longer than the crush relief length CL of the crushrelief 73 of the corresponding half bearing 7.

In addition, in the half thrust bearing 8 of the embodiment, the thrustrelief length L1 of the front side thrust relief 82F at the inner endportion is shorter than the crush relief length CL (FIG. 18) of thecrush relief 73 of the corresponding half bearing 7.

Here, the “crush relief length CL” of the crush relief 73 means a lengthof the crush relief 73 disposed correspondingly to the position of therear side thrust relief 82R or the position of the front side thrustrelief 82F at the axial end portion on the side where the half thrustbearing 8 is provided (FIG. 4). In particular, the crush relief lengthCL is expressed by, when the half bearing 7 is placed on a horizontalsurface such that both circumferential end surfaces 74 and 74 become alower end surface, a height from the horizontal surface to an upper edgeof the crush relief 73 (FIG. 18). It is to be noted that the crushreliefs 73 at both circumferential end portions of the half bearing 7have the same crush relief length. Alternatively, unlike the embodiment,the crush relief length of the crush reliefs 73 of the half bearing 7may be varied in the axil direction of the half bearing 7.

Hereinafter, the operation of the present invention will be described.

Immediately after flowing out of the clearance between the crush reliefsof the half bearings 7, the lubrication oil flows on a front side in thecrankshaft rotational direction from the position of the abutmentportion (joint portion) of a circumferential end surface of one halfthrust bearing 8 and that of the other half thrust bearing 8 as shown inFIGS. 17A and 17B (dashed arrow) due to an inertial force which causesthe oil to move toward a front side in the rotational direction of thecrankshaft 11 because the lubrication oil has circumferentially flownwith the surface of the journal portion of the rotating crankshaft.

The thrust bearing 10 of the bearing device 1 according to theembodiment includes the pair of half thrust bearings 8 and 8, and thethrust relief length L2 of the rear side thrust relief 82R of each halfthrust bearing 8 is longer than the thrust relief length L1 of the frontside thrust relief 82F. Further, in the half thrust bearing 8, thethrust relief length L2 of the rear side thrust relief 82R at the innerend portion is longer than the crush relief length CL of the crushrelief 73 of the corresponding half bearing 7, and the thrust relieflength L1 of the front side thrust relief 82F at the inner end portionis shorter than the crush relief length CL of the crush relief 73 of thecorresponding half bearing 7. According to the configuration, the centerof the length at the inner end portion of a thrust relief clearance Ssurrounded by the surface of the front side thrust relief 82F of onehalf thrust bearing 8, the surface of the rear side thrust relief 82R ofthe other half thrust bearing 8, and the imaginary sliding surfaceobtained by extending the sliding surface 81 over the surfaces of thefront side thrust relief 82F and the rear side thrust relief 82R (seeFIG. 17B) is positioned on a front side in the rotational direction ofthe crankshaft 11 relative to the abutment portion (joint portion) ofthe circumferential end surface 83 of one half thrust bearing 8 and thatof the other half thrust bearing 8. Accordingly, the lubrication oilflown out of the crush relief clearance of the half bearings 7 andflowing toward a front side in the rotational direction of thecrankshaft 11 from the position of the abutment portion (joint portion)of the circumferential end surfaces 83 and 83 of the pair of half thrustbearings 8 flows into the thrust relief clearance S in large amounts,and thus the large amount of lubrication oil can be fed to the slidingsurface 81.

During the operation of the internal combustion engine, the deformationof the crankshaft (in the axial direction) occurs especially under theoperation condition in which the crankshaft is rotated at high speeds,and therefore the vibration of the crankshaft becomes large. The largevibration periodically generates the axial force f in the crankshafttoward the sliding surface 81 of the thrust bearing 10. The slidingsurface 81 of the thrust bearing 10 receives the axial force f.

In the thrust bearing 10 of the bearing device 1 according to thepresent invention, even under the axial force f of the crankshaft, thelubrication oil flowing out from the crush relief clearance between thepair of half bearings 7 easily flows in large amounts into the thrustrelief clearance S formed by the thrust relief 82F positioned on a frontside in the rotational direction and the thrust relief 82R positioned ona rear side in the rotational direction in the thrust bearing 10, andthe large amount of lubrication oil is fed to the sliding surface 81 asdescribed above. Because the large amount of lubrication oil issupplied, the sliding surface 81 of the thrust bearing 10 hardly comesinto direct contact with the surface of the thrust collar 12 of thecrankshaft. Also, the thrust bearing 10 of the bearing device 1according to the present invention still has a high ability of receiving(bearing) the axial load of the crankshaft since the area of the slidingsurface 81 is not reduced.

As specific dimensions of the crush relief 73 of the half bearing 7, adepth of the crush relief 73 at the end surface 74 measured from animaginary extended surface obtained by extending the sliding surface 75over the crush relief is 0.01 to 0.1 mm, and the crush relief length CLis 3 to 7 mm when used for a crankshaft (in which a journal portiondiameter is about 30 to 100 mm) of a small internal combustion enginefor a passenger car, for example.

Also, the thrust relief length L2 of the rear side thrust relief 82R ofthe half thrust bearing 8 at the inner end portion preferably satisfiesthe formula: L2≧1.5×CL with respect to the crush relief length CL of thecrush relief of the correspondingly positioned half bearing 7. Further,the thrust relief length L1 of the front side thrust relief 82F of thehalf thrust bearing 8 at the inner end portion preferably satisfies theformula: L1≦1.5×CL with respect to the crush relief length CL of thecrush relief 73 of the correspondingly positioned half bearing 7.

In the embodiment, the description has been given for the bearing device1 using such a type of thrust bearing 10 that the half bearing 7 and thehalf thrust bearing 8 are separated from each other, however, thepresent invention is not limited to this. The present invention alsoencompasses a bearing device 1 using such a type of thrust bearing 10that the half bearing 7 and the half thrust bearing 8 are integrated.

Embodiments 1-5 of the present invention has been described in detailabove with reference to the drawings, however, the specificconfigurations are not limited to these embodiments, and the presentinvention includes design modifications which do not depart from thegist of the present invention.

For example, as shown in FIG. 14, the present invention may be appliedto a thrust bearing using a half thrust bearing with a protrusionprotruding radially outwardly for positioning and rotation prevention.Also, the circumferential length of the half thrust bearing may beshorter than that of the half thrust bearing 8 shown in embodiment 1,for example, by a predetermined length S1. Also, the half thrust bearing8 may have an arcuate cutout having a radius R on its innercircumferential surface close to the circumferential end surfaces.

When forming the arcuate cutout with the radius R in this manner, thethrust relief lengths L1 and L2 and the thrust relief depths RD of thethrust reliefs 82F and 82R can be expressed by the lengths withreference to extension lines of upper edges of the thrust reliefs 82Fand 82R and the depths with reference to extension surfaces of surfacesof the thrust reliefs 82F and 82R, respectively, that are obtained whenthe arcuate cutout is not formed.

Similarly, the half thrust bearing 8 may have chamfers along thecircumferential direction at the radial outer or inner edge on thesliding surface side. In that case, the thrust relief lengths and thethrust relief depths at the inner end portion and the outer end portioncan be expressed by thrust relief lengths and thrust relief depths atpositions of inner and outer diameter side end portions of the halfthrust bearing, that would exist when the chamfers is not formed.

Also, in order to prevent misassemble, at only one of two abutmentportions of a pair of half thrust bearings 8, circumferential endsurfaces of the half thrust bearings 8 may be formed as inclined endsurfaces 83B as shown in FIG. 15, so that the inclined end surfaces formthe abutment portions. In that case, the inclined end surfaces 83B areinclined by a predetermined angle θ3 with respective to a plane passingthrough the other circumferential end surfaces which are not inclined(actual split plane H). Alternatively, the circumferential end surfacesmay have other shapes such as corresponding convex and concave shapesinstead of the inclined end surfaces 83B.

In any case, those skilled in the art would understand that the thrustrelief lengths L1 and L2 are defined as vertical lengths from the actualsplit plane H to the point where the surfaces of the thrust reliefs 82RBand 82FB intersect the inner edge of the sliding surface 81B.

As shown in FIG. 16, oil grooves 81 a may be formed on a sliding surface81 of a half thrust bearing 8D such that the oil grooves 81 a have thesame shape as recesses 182 formed at the abutment portions by pairingthe front side thrust reliefs 82F and the rear side thrust reliefs 82Rof the present invention. It is to be noted that, although one halfthrust bearing has two oil grooves 81 a on the sliding surface 81 in theembodiment, the present invention is not limited to this, and the halfthrust bearing may have one, or three or more oil grooves. Also,although the oil grooves 81 a in FIG. 16 are illustrated as recesses ofa shape having the thrust relief clearance S formed in the thrustbearing 10C shown in FIG. 10 as an example, the shape of the oil grooves81 a is not limited to this.

Above embodiments have been described for the case where one bearingdevice 1 uses two thrust bearings 10 of the present invention, however,the present invention is not limited to this. The desired effect canalso be obtained by using the thrust bearing 10 of the present inventionon either of the axial end surfaces of the bearing housing and a thrustbearing with thrust reliefs of a conventional or other configuration onthe other end surface. Alternatively, the thrust bearing 10 of thepresent invention may be a pair of bearing in which the half thrustbearings 8 constituting the thrust bearing 10 are integrally formed withthe half bearings 7 on one or both axial end surfaces thereof.

The invention claimed is:
 1. A thrust bearing for receiving an axialforce of a crankshaft of an internal combustion engine, comprising apair of half thrust bearings each having a semi-annular shape, the pairof half thrust bearings forming an annular shape by butting respectivecircumferential end surfaces thereof, wherein each half thrust bearingcomprises a first thrust relief on a front side in a crankshaftrotational direction, a second thrust relief on a rear side in thecrankshaft rotational direction, and a sliding surface arranged betweenthe first and second thrust reliefs for receiving an axial force, thefirst and second thrust reliefs being formed so that a wall thickness ofthe half thrust bearing is made thinner toward the circumferential endsurface, and a second thrust relief length of the second thrust reliefpositioned on a rear side of one of the half thrust bearings in acrankshaft rotational direction is larger than a first thrust relieflength of the first thrust relief positioned on a front side of theother of the half thrust bearings in the crankshaft rotationaldirection; wherein the first thrust relief length extends from a firstone of the circumferential ends to where the first thrust relief meetsthe sliding surface; and wherein the second thrust relief length extendsfrom a second one of the circumferential ends to where the second thrustrelief meets the sliding surface.
 2. The thrust bearing according toclaim 1, wherein a thrust relief depth at the circumferential endsurface of the second thrust relief positioned on the rear side of theone of the half thrust bearings in the crankshaft rotational directionand a thrust relief depth at a circumferential end surface of the firstthrust relief positioned on the front side of the other of the halfthrust bearings in the crankshaft rotational direction are identicalwith each other in a radial direction of a butting portion of thecircumferential end surfaces of the pair of half thrust bearings.
 3. Thethrust bearing according to claim 1, wherein a thrust relief depth ofeach one of the first and second thrust reliefs at the circumferentialend surfaces is constant in the radial direction of the first and secondthrust reliefs.
 4. The thrust bearing according to claim 1, wherein atleast the second thrust relief positioned on the rear side of each halfthrust bearing in the crankshaft rotational direction consists of a flatsurface or a curved surface.
 5. The thrust bearing according to claim 1,wherein at least the second thrust relief positioned on the rear side ofeach half thrust bearing in the crankshaft rotational direction consistsof two flat surface portions, two curved surface portions, or a flatsurface portion and a curved surface portion.
 6. The thrust bearingaccording to claim 1, wherein the second thrust relief positioned on therear side of each half thrust bearing in the crankshaft rotationaldirection consists of a first curved surface portion adjacent to thecircumferential end surface of the half thrust bearing and a flatsurface portion adjacent to the sliding surface, the first curvedsurface portion being a convex curved surface bulging from a slidingsurface side toward a back face side as seen in a radial direction. 7.The thrust bearing according to claim 1, wherein the second thrustrelief positioned on the rear side of each half thrust bearing in thecrankshaft rotational direction consists of a first curved surfaceportion adjacent to the circumferential end surface of the half thrustbearing and a second curved surface portion adjacent to the slidingsurface, the first curved surface portion being a convex curved surfacebulging from a sliding surface side toward a back face side as seen in aradial direction, the second curved surface portion being a convexcurved surface bulging from the back face side toward the slidingsurface side.
 8. The thrust bearing according to claim 1, wherein athrust relief depth of each one of the first and second thrust reliefsat the circumferential end surfaces is maximum at a radially inner endof the first and second thrust reliefs and is made shallower towards aradially outer end of the first and second thrust reliefs.
 9. The thrustbearing according to claim 1, wherein the second thrust relief length ofthe second thrust relief positioned on the rear side of each half thrustbearing in the crankshaft rotational direction is minimum at a radiallyinner end of the first and second thrust reliefs and is made longertowards a radially outer end, and the first thrust relief length of thefirst thrust relief positioned on the front side of each half thrustbearing in the crankshaft rotational direction is constant from theradially inner end to the radially outer end of the first and secondthrust reliefs.
 10. The thrust bearing according to claim 9, wherein thesecond thrust relief positioned on the rear side of each half thrustbearing in the crankshaft rotational direction consists of a firstcurved surface portion adjacent to the circumferential end surface ofthe half thrust bearing and a flat surface portion adjacent to thesliding surface, the first curved surface portion being a convex curvedsurface bulging from a sliding surface side toward a back face side asseen in a radial direction.
 11. The thrust bearing according to claim 9,wherein the second thrust relief positioned on the rear side of eachhalf thrust bearing in the crankshaft rotational direction consists of afirst curved surface portion adjacent to the circumferential end surfaceof the half thrust bearing and a second curved surface portion adjacentto the sliding surface, the first curved surface portion being a convexcurved surface bulging from a sliding surface side toward a back faceside as seen in a radial direction, the second curved surface portionbeing a convex curved surface bulging from the back face side toward thesliding surface side.
 12. The thrust bearing according to claim 1,wherein the second thrust relief length of the second thrust reliefpositioned on the rear side of each half thrust bearing in thecrankshaft rotational direction is constant from a radially inner end toa radially outer end of the first and second thrust reliefs, and thethrust relief length of the first thrust relief position on the frontside of each half thrust bearing in the crankshaft rotational directionis constant from the radially inner end to the radially outer end of thefirst and second thrust reliefs.
 13. A bearing device comprising: acrankshaft of an internal combustion engine; a pair of half bearings forsupporting a journal portion of the crankshaft, crush reliefs beingformed on an inner circumferential surface of each half bearing andadjacent to both circumferential ends of the half bearing; a bearinghousing having a retaining hole for retaining the pair of half bearings,the retaining hole being formed to penetrate through the bearinghousing; and at least one thrust bearing according to claim 1, thethrust bearing being disposed at an axial end surface of the bearinghousing.
 14. The bearing device according to claim 13, wherein thesecond thrust relief on the rear side of each half thrust bearing in thecrankshaft rotational direction is formed so that the second thrustrelief length at a radially inner end is longer than a crush relieflength of the crush relief of a corresponding axial end portion of thehalf bearing.
 15. The bearing device according to claim 14, wherein aformula: L2≧CL×1.5 is satisfied, where the second thrust relief lengthat a radially inner end of the second thrust relief positioned on therear side in the crankshaft rotational direction is L2, and the crushrelief length of the crush relief at the corresponding axial end portionof the half bearing is CL.
 16. The bearing device according to claim 14,wherein the first thrust relief positioned on the front side of eachhalf thrust bearing in the crankshaft rotational direction is formed sothat the first thrust relief length at a radially inner end is shorterthan a crush relief length of the crush relief at a corresponding axialend portion of the half bearing.
 17. The bearing device according toclaim 16, wherein a formula: L1≦CL×1.5 is satisfied, where the firstthrust relief length at a radially inner end of the first thrust reliefpositioned on the front side in the crankshaft rotational direction isL1, and the crush relief length of the crush relief at the correspondingaxial end portion of the half bearing is CL.